Composition adapted for inductive heating and method for using same



Patented Jan. 22, 1946 COMPOSITION ADAPTED FOR INDUCTIVE HEATING AND METRO D FOR USING SAME Fred Kohler, New York, N. Y., assignor to Induction Heating Corp., New York, N. Y.,

tion of NewYork a corpora- No Drawing. Application May 21, 1943, Serial No. 487,984

16 Claims.

This invention relates to compositions and materials adapted to be heated by high frequency magnetic induction and to methods for inductively heating non-conductive material. Among other possible uses, the invention is particularly adapted for the inductive heating of adhesives, plastics and other non-conductive materials, which in the course of manufacturing processes, are required to be heat-treated within a predetermined limited temperature range to effect polymerization, curing or softening for various molding or adhesive purposes.

Methods utilizing electrostatic fields have recently been perfected for heating assemblies of dielectric material which are to be secured together by thermosetting phenolic resin glues and the like. A layer or film of the glue is, for example, interposed between two pieces of wood or other dielectric material, and the assembly then subjected to an electrostatic field of extremely high frequency, the assembly being held together under pressure until the glue is heated and thereby hardened. This process requires considerable time, in that not only the glue but also the objects to be secured together are necessarily heated dielectrical- 1y up to the desired thermosetting temperature for the glue. Also there is a considerable waste of power in heating the whole assembly, and in some cases, as where the objects are of wood for example, the heating thereof may cause excessive drying or other undesirable modification.

Since objects such as wood and the like, and the adhesives available for securing same together, are non-conductive and also non-magnetic, they cannot be satisfactorily heated through high frequency magnetic induction by any method heretofore available so far as I am-aware. I have discovered, however, that upon mixing with the glue, adhesive, plastic or other non-conductive material, finely divided magnetic particles of the ferromagnetic class, it is possible to very quickly heat the mixture by magnetic induction, 1. e., by subjecting the mixture to a high frequency electromagnetic field, which quickly heats the magnetic particles because of the hysteresis effect therein, and consequently the hot particles in turn quickly heat the associated non-magnetic material to the desired temperature. Iron filings, for example, might be used as the magnetic particles, but since iron particles become quickly heated to redness' in a strong high frequency magnetic field, it is undesirable to use iron for the purpose, unless the non-magnetic material -to be heated i capable of withstanding the temperatures of red hot iron particles without injury. Since it is too difiicult in practice to uniformly control and limit the intensity of high frequency magnetic fields as applied to all of the particles, it is not practical to use iron filings for the purpose with the usual glues and adhesives.

I have discovered, however, that by properly selecting other kinds of finely divided metal particles and alloy having ferromagnetic properties, the heating temperature in the presence of a high frequency magnetic field may be readily limited and controlled to the particular temperature or temperature range necessary for the heat-treating of materials such as glue, adhesives or plastics. As is well-known, when any of the various metals or alloys of the ferromagnetic class become heated to a particular temperature, known as the Curie point therefor, then the ferromagnetic qualities cease. As a result, any further application of a high frequency field is substantially ineffective to cause further heating if the particles are small. Masses of metal of substantial size may be heated by magnetic induction due to the setting up of eddy currents, as well asbecause of hysteresis effects in the case of metals of the ferromagnetic class. In accordance with my invention, however, the ferromagnetic particles being finely divided and effectively insulated from each other by the adhesive or other dielectric material mixed therewith, there is no substantial heating above the Curie point due to eddy currents. Accordingly the heating effect is substantially confined to that resulting from the effects of hysteresis, and consequently is limited to the Curie temperatures for the particular magnetic particles used. Thus a layer, coating, or film of adhesive, or a mass of plastic material when mixed or associated with particles having a properly selected Curie point, may be heated by magnetic induction to the exact temperature or temperature range necessary for curing. molding or softening the non-conductive material without danger of overheating or burning the same. In fact, particles may be chosen such that the heating effect is discontinued upon reaching the Curie point for the particles, so that the non-conductive material is protected against injury if it is of a character which might be injured merely by more prolonged heating without a further temperature rise.

Various other features, objects and advantages of the invention will be apparent from the following more detailed description. The invention resides in the novel compositions, methods and combinations of methodsteps hereinafter described by way of examples, and as will be more particularly pointed out in the appended claims.

The invention has been successfully carried out for gluing together articles of wood with phenolic formaldehyde resins of the thermosetting types heretofore used in connection with dielectric heating. That is, the proper normal thermosetting temperature range for such resin glues is first determined, and then the glue is mixed with a substantial amount of finely divided metal particles of the ferromagnetic class, and selected as having a Curie point within or not excessively above such normal thermosetting range. This mixture is then interposed in the usual way between the articles which are to be secured together. The assembly may be clamped or otherwise held together under pressure, as heretofore, and then subjected to a high frequency electromagnetic field. Because of hysteresis effects, this will cause the magnetic particles to be quickly brought up to the Curie temperature. Consequently the resin or adhesive material associated or mixed therewith will also be promptly heated to the desired curing temperature, and no higher. All danger of burning or destructive decdmposition is thus avoided, even though the high frequency field is continued indefinitely. The articles of wood or other dielectric material which are to be secured together, moreover will not be subjected to any heating effect except temporarily at the surfaces immediately adjacent the magnetic particles. Thus the heating effect is confined to the seam or joint, and heating of the entire assembly, as occurs with the dielectric heating method, is avoided with a substantial saving of power and with avoidance of drying out or otherwise injuring the articles being secured together. With a given amount of available power, there is also a substantial reduction in the time required for the heating step.

In carrying out one example of the invention, well-known phenolic formaldehyde resin glues were used in a semi-liquid state, available on the market under the trade name Catabond No. 400 or No. 590. These were mixed with finely divided nickel-copper alloy (20% copper and 80% nickel) in a proportion of approximatelv 30% of the metal powder to 70% of the glue. Wooden articles to be secured together were coated with this resin alloy mixture and then while being pressed together, were subjected to a high frequency magnetic field sufficient to heat the metal alloy particles up to their Curie temperature, which in this case is approximately 170 C. This caused the glue to polymerize and harden in about -12 minutes or less, without heating the adjacent bodies of wood appreciably, the heat being generated directly in the glue mixture and substantially confined thereto. The high frequency induction heating equipment was of a type now commonly used for induction heating purposes such as brazing and heat-treating of metal, the generator having a capacity of 20 kilowatts and a frequency of in the neighborhood of 375,000 cycles. For assemblies of considerable size however, a 2-kilorwatt generator will be found sufficient. The frequency is not critical and may be varied from several thousand to several hum dred thousand. The work coils and associated equipment by which the high frequency field is applied to the assembly, may for example be of types such as disclosed in U. S. patents to Goodridge, No. 2,308,240 and to Dravneek, No. 2,321,189. The type and shape of work coil used will of course depend upon the external shape of the assembly being glued together and in general for most rapid and economical results, the turn or turns of the coil should be so shaped as to come as close as possible to the layer of glue mixture being heated.

The relative proportions of the magnetic particles and plastic or adhesive material, are not critical and may vary considerably depending upon the characteristics of theadhesiv or plastic and the amount of high frequency power available, and the desired.time of treatment. In general it has been found desirable that the mixture contain in the neighborhood of about 20 to of magnetic particles or alloy.

While the degre of fineness of the particles is not critical, it is believed that for most purposes, the best results may be obtained by using particles or powder of. very fine mesh, for example 200-300 mesh or less, which will assure the maintenance of good suspension in the mixture.

It will be apparent that the various features of the invention are applicable for use with adhesives, resins and plastics other than the particular examples above given and with materials which are thermoplastic, as well as those which are thermosetting, and with both organic and inorganic adhesives which are adapted to be rendered adhesive by heating. The invention may also be used in ways other than by mixing the magnetic particles with liquid or semi-liquid nonmagnetic material. For example, films of thermoplastic or thermosetting compositions may be used with the magnetic particles sprinkled over the surface thereof or impressed into the surface, or the particles may be interposed between two or more such films. Plastics or resins within or upon which the particles may be used according to the invention, include formaldehyde urea, Glyptal, vinyl resins, cellulose acetate or nitrate resins, and methacrylate resins.

Films of plastic material have also been secured together in accordance with one phase of the invention, by interposing therebetween an area of wire screening, and then subjecting to a high frequency magnetic field. That is, the metal of which the wire is formed was selected from those herein disclosed as having a suitably low Curie point. If the wire is of small diameter, heating due to eddy currents is minimized.

In some manufacturing operations, such as in the assembly and gluing together of wooden objects, it may be found desirable to use the above types of alloy resin mixtures with high frequency magnetic induction applied only at intervals along the joint. In this way, what may be termed spot gluing may be quickly accomplished so that the securing clamps may then be taken off for prompt reuse. Thereafter, if desired, the remaining areas of glue may be heated either dielectrically or by high frequency magnetic induction, or otherwise cured if desired over a longer time interval.

The invention affords a convenient means for securing objects together in such a way that they may be again readily and quickly separated by heating the adhesive layer without substantially heating the objects. For this purpose, vari- 2,808,841 ous well-known thermoplastic materials may be separated while the interposed thermoplastic is heated to its softening temperature.

The invention may be used with the various types of inorganic material such as sodium or calcium silicate adhesives of known types, mixed with finely divided particles having a Curie point at the temperature customarily used for the application of such adhesives under heat.

As is well known, certain alloys of the ferromagnetic class, such as various alloys of manganese, when once heated up to the Curie temperature, do not reacquire their ferromagnetic properties until after they have been cooled to a temperature very substantially below the Curie point. For example, in the case of certain manganese alloys, the ferromagnetic properties are not reacquired until the alloy is cooled far below room temperature. The use of metal particles of alloys of this type is advantageous with the present invention in conjunction with adhesives or other non-magnetic material which might be injured by any prolonged heating even at temperatures below the Curie point or by repeated heating up to the Curie point 'of the metal particles. The same property may also be utilized to insure against any injury to the particular adhesive used in case a finished assembly should accidentally be repeatedly subjected to a high frequency magnetic field. That is, the initial application of the high frequency field for heating the adhesive up to the Curie point of the metal particles will terminate the generation of heat in the assembly so that it cannot be inductively heated any longer, nor heated again inductively, by high frequency, unless the assembly is first cooled to a very low temperature for reestablishing the ferromagnetic properties of the metal particles. By using such particles the power output or load of the high frequency generator discontinues automatically upon the initial heating to the Curie point.

The principles of the invention may be used in connection with molding or shaping operations, by mixing the metal particles with plastics adapted to be polymerized, hardened or otherwise cured or shaped upon heating in a mold. In this case the mold may be formed of a non-conductive material whereby the mixture in the mold may be subjected to a high frequency field by placing a high frequency work coil adjacent the exterior of the mold.

Listed below are numerous examples of metals and alloys having various relatively low Curie points and thus particularly adapted for use as the magnetic particles in accordance with the invention. Those having the higher Curie points, of course, are not adapted for use with organic adhesives. The invention is not intended to be confined to the use of these particular illustrative examples.

Nickel-chromium alloys chromium (remainder nickel) 2 M Heusler alloys (manaanese-aluminum-copper) Manganese, Aluminum Copper Curie point, per cent per cent per cent degrees C.

13.9 7. 2 78. 9 205 6 l3. 2 80. 8 305 26. 5 l6. 3 57. 2 525 11. ii 25. 0 63. 4 5-10 l Nickel-copper alloys P Curie point, er cent copper (remainder nickel) degrees Q Nickel-manganese alloys Per nt Curie point,

cc manganese (remainder nickel) degrees C.

Per cent aluminum (remainder nickel) Curie point, degrees C.

From the above it will be apparent that a wide variety of alloys may be selected having the proper Curie temperature for heat-treating any of the commonly used adhesives, plastics, etc. If higher temperature ranges are desired, it is of course possible to use particles of iron, cobalt or alloys thereof. Iron has a Curie point of 770 C., and cobalt 1125 C. Particles of these metals are thus adapted for or fire clay mixtures for example, to form stove plates or the like, capable of being heated to limited predetermined temperatures by high frequency magnetic induction.

While the invention has been described in detail with respect to certain preferred examples, it will be understood by those skilled in the art after understanding the invention that various changes and modifications may be made without departing from the spirit and scope of the invention, and it is intended thereforein the appended claimsto cover all such changes and modifications.

What is claimed as new and desired to be secured by Letters Patent is:

1. Method for securing together objects which are substantially non-conductive electrically, which comprises interposing therebetween an electrically non-conductive adhesive which is thermosetting within a predetermined temperature range but subject to destructive decomposiincorporation in asbestos, mica tion at a higher temperature, also interposlng therewith finely divided distributed magnetic particles having a Curie point below the latter temerature but at a temperature within or above said range, and heating said particles and consequently heating said adhesive to temperatures substantially limited to said Curie point, by subjecting said particles to a high frequency magnetic field, said particles being chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is avoided to an extent that will avoid heating said adhesive to said higher temperature.

2. Method for curing a compound which is electrically substantially non-conductive, by inductive heating to a limited predetermined temperature below the temperature of injurious decompositlon of the compound, which comprises mixing therewith distributed metal particles comprising nickel, having magnetic properties essentially due to the presence of the nickel and a. Curie point substantially approximating said temperature, and heating such metal by subjecting same to a high frequency magnetic field and thereby heating the associated non-conductive material, said particles being chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is substantially avoided.

3. Method for inductively heating material which is electrically substantially non-conductive, to a limited predetermined temperature below the temperature of injurious decomposition of the compound, which comprises associating therewith distributed ferromagnetic alloy particles selected as having a Curie point substantially approximating said temperature, and heating such metal to a temperature substantially limited by said Curie point, by subjecting same to a high frequency magnetic field and thereby heating the associated non-conductive material, said particles bein chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is substantially avoided.

4. Method for inductively heating material which is electrically substantially non-conductive and subject to injury at a high temperature, which comprises associating said material with distributed particles of a magnetic material having a Curie point below said temperature, and heating said magnetic material by high frequency magnetic induction and thereby heating the associated non-conductive material to a temperature substantially limited to the Curie point of said magnetic material, said particles being chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is substantially avoided.

5. Method for inductively heating material which is electrically substantially non-conductive and subject to injury at a high temperature, which comprises associatin said material with finely divided distributed particles of a magnetic material having a Curie point below said temperature, said magnetic material being selected from the group consisting of Heusler alloys, nickel, and nickel alloyed with chromium, copper, manganese or aluminum, and heating said magnetic material by high frequency magnetic induction and thereby heating the associated non-conductive material to a temperature substantially limited to the Curie point of said magnetic material, said particles being chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is avoided to an extent aaeauu that will avoid heating said material to said hich temperature.

6. Method for heating'a material which is electrically substantially non-conductive, to a predetermined limited temperature without injury from overheating, which comprises associating therewith distributed finely divided metal particles selected as having a Curie point approximatinfl said temperature, and subjecting to a high frequency magnetic field until said particles are heated to said Curie point, said particles being chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is substantially avoided.

7. Method for securing together objects which are substantially non-conductive electrically, which comprises interposing therebetween a film carrying distributed particles having ferro-magnetic properties and also comprising material effective as an adhesive upon heating, said particles having a Curie point below the temperature of destructive decomposition of said material, and heating said particles to a temperature limited by said Curie point, by subjecting same to a high frequency magnetic field, and consequently heating said material, said particles being chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is substantially avoided.

8. Method for securing together objects which are substantially or largely non-conductors electrically and are subject to destructive decomposition upon heating above a predetermined temperature, which comprises bonding the objects together by an interposed thermoplastic layer associated with distributed particles having ferromagnetic properties and a Curie point below said temperature, whereby the objects may be separated without such decomposition upon subject ing to a high frequency magnetic field for heating said particles to said Curie point and consequent heating and release of the thermoplastic, said particles being chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is substantially avoided.

9. Method for inductively heating material which is electrically substantially non-conductive and subject to injury at a high temperature, which comprises associating said material with metal having ferro-magnetic properties and a Curie point below said temperature, and heating said metal by high frequency induction and thereby heating the associated non-conductive material to a temperature substantially limited to said Curie point, said metal being in the form of separated portions, two dimensions of which are so small that with the high frequency used, eddy current heating above said Curie point is avoided to an extent that will avoid the injurious high temperature.

10. Method for inductively heating fluid material which is substantially or largely non-conductive electrically and which is subject to injury at a high temperature, which comprises formin a suspension therein of distributed particles having ferro-magnetic properties and a Curie point below the injurious high temperature, and subjecting said particles to a high frequency magnetic field of suilicient strength and duration to heat said particles and fluid material to the desired temperature limited by said Curie point, said particles being chosen in such small dimensions that with the high frequency used, eddy current heating above said Curie point is substantially avoided.

11. Method for curing an organic compound.

heating said material to which comprises associating therewith distributed term-magnetic metal particles having a Curie point below the temperature of destructive decomposition of the compound, and heating such particles to a temperature limited to substantially said Curie point, by subjecting same to a hig frequency magnetic held and thereby heating the associated compound, said particles being chosen of such small dimensions that with the high frequency used, eddy current heating above said Curie point is substantially avoided.

12. Method for inductively heating material which is electrically substantially non-conductive and subject to injury at a high temperature, which comprises associating said material with metal having ierro-magnetic properties and a Curie point below said temperature, and heating said metal by h gh frequency magnetic induction and thereby heating the associated non-conductive material to a temperature substantially limited to said Curie point, said metal being in the form of pieces of fine wire distributed through the material and oi cross-sectional dimensions so small that with the high frequency used, eddy current heating above said Curie point is avoided to an extent that will avoid heating said material to an injurious high temperature.

13. Method for inductively heating material which is electrically substantially non-conductive. for a limited time and to a limited predetermined temperature below the temperature of injurious decomposition of said material, which comprises associating therewith metal selected as having a Curie point substantially approximating said temperatureand in which after heating to such Curie point the ferro-magnetic properties are not substantially restored until after substantial coolin and heating such metal to a temperature substantially limited to said Curie point, by subjecting same to a high frequency magnetic field and thereby heating the associated non-conductive material, said metal being in the form of portions distributed through the material and having dimensions so small that with the high frequency used, eddy current heating above said Curie point is avoided to an extent that will avoid heating said material substantially above said limited predetermined temperature.

14. A composition adapted for use as an adhesive upon heating by high frequency magnetic induction, comprising an electrically substantially non-conductive organic material normally eii'ective as an adhesive upon heating to within a predetermined temperature range but subject to destructive decomposition at a higher temperature, and finally divided and distributed particles hav ing term-magnetic properties associated therewith, said particles having a Curie point within or above said predetermined temperature range but below said temperature of destructive decomposition, the proportion of the composition which constitutes non-conductive material being sufllcient to maintain the composition as a whole nonconductive to a degree which will avoid eddy current heating to temperatures substantially above said Curie point, when the composition is sub jected to a high frequency magnetic field.

15. In combination with a liquid adhesive material which is electrically substantially non-conductive and is rapidly polymerizable to a solid upon heating to within a predetermined temperature range and subject to destructive decomposition at a higher temperature, finely divided particles having ierro-magnetic properties mixed therewith and effectively insulated from each other thereby, said particles having a Curie point below said higher temperature although within or above said range, whereby said material is pol merizable without danger of injury from overheating, upon subjecting to a high frequency magnetic field.

16. An article of manufacture comprising a pair of members formed of electrically substantially non-conductive material, and a layer of adhesive composition therebetween securely retaining said members against relative movement, said adhesive composition comprising an adhesive material which is also electrically substantially non-conductive and embodying therein finely divided and distributed particles having ferromagnetic prop- 40 erties, said particles having a Curie point below the temperature of destructive decomposition of either of said members or of said adhesive material, the relative proportions of the adhesive and of said particlesin the composition being such as to maintain the composition as a whole non-conductive to a degree which will avoid eddy current heating to temperatures substantially above said Curie point when the composition is subjected to a high frequency magnetic field.

FRED KOHLER. 

